1. Field of the Invention
The present invention relates to the deposit of an aluminum coating on a metal part, notably on a hollow metal part comprising a cavity with an opening allowing access to this cavity. Its object is more particularly the application of such a coating on hollow blades of a turbomachine incorporating a cooling fluid distribution sleeve.
2. Discussion of the Background
A gas turbine engine, as used for propulsion in the aviation field, comprises an atmospheric air inlet which communicates with one or more compressors, usually including a fan, rotated about one and the same axis. The main flow of this air after having been compressed supplies a combustion chamber placed annularly about this axis and is mixed with a fuel in order to provide hot gases downstream to one or more turbines through which the latter are expanded, the turbine rotors driving the compression rotors. The engines operate at a turbine inlet driving gas temperature that is sought to be as high as possible because performance is related thereto. For this purpose, the materials are selected to withstand these operating conditions and the walls of the parts swept by the hot gases, such as the upstream guide vanes or the movable turbine fins, are provided with cooling means. Furthermore, because of their metal constitution made of nickel-based or cobalt-based superalloy, it is also necessary to protect the latter from the erosion and corrosion caused by the components of the driving gases at these temperatures.
A known method for protecting these parts is to deposit an aluminum-based coating on the surfaces prone to attack by the gases. Aluminum attaches to the substrate by metal interdiffusion and forms an oxide-protective layer on the surface. The thickness of the protective layer is of the order of a few tens of microns.
The present invention relates to the technique, known per se, of depositing aluminum in the vapor phase or else called aluminization by deposit in the vapor phase. According to the method, the parts to be treated are placed in an enclosure in which the atmosphere consists of a mixture of an inert or reducing gas, for example argon or hydrogen, and an active gas comprising an aluminum halide. At the reaction temperature, between 900° C. and 1150° C., the halide decomposes on the surface of the part into gaseous halogen and aluminum which diffuses into the metal.
The halide is produced by placing in the enclosure with the parts to be treated metal blocks of aluminum or of aluminum alloy which form the donor, in the presence of granules of a halogen, chlorine or fluorine compound, which form the activator. The inert gas is made to travel over the activator at a temperature allowing the sublimation of the halogen which is carried to the donor and with which it reacts to produce the metal halide which, at this temperature, is in vapor form. The halide then decomposes on contact with the metal substrate to be coated allowing the aluminum to be deposited; the gaseous halogen is reformed.
When the stator parts also, although mobile, are provided with inner cavities swept by a cooling fluid, air drawn from the compressor, it has been noted that the walls of these cavities were also subject to corrosion. Returns of parts used on engines working in certain environments have shown an attack of these surfaces. For example, internal corrosion of the upstream guide vanes, releases of flakes of corrosion in the cavity of the upstream guide vanes, the blocking of the trailing edge vent holes, etc. have been noted. Protection for these portions of parts is therefore also necessary.